Take the word “First” in most slavic languages. In russian-Pervy, Slovak and south Slavic languages- prvy, and then Polish Perwszy.
In Ukrainian and eastern Slovak: peršij, perši…
I think Nobility in poland must have added these sounds to sound fancy and it spread to the rest of the country.
I read somewhere (maybe here on Slavorum) that for example Ł was introduced by peasants and the nobility thought it sounded bad and considered it as peasant language. They used normal L. But it spread to other parts of Poland and it’s part of their standard alphabet now. So maybe it was the other way around.